Elevator systems are typically guided between a pair of ferrous rails, such as steel, which are also used as braking surfaces for emergency stops. In normal operation, all of the motion of the elevator and all of the arresting of that motion is caused by the hoist ropes, which are moved upwardly and downwardly, or held in a fixed position by means of a sheave, the motion of the sheave being controlled by the elevator drive motor and the machine brake which are mechanically coupled to the sheave. Machine brakes typically are spring actuated into the braking position against a drum or a disk attached to the sheave, and use electromagnets to release the brakes from the braking position when the elevator is to move. This provides fail-safe braking insofar as electrical power or electronic signaling is concerned.
A governor rope is attached to the elevator and rotates a governor, at a rate of rotary speed that relates to the elevator's linear speed, which has fly weights that move outwardly with increasing speed as a result of centrifugal force. When the elevator exceeds its rated speed (sometimes called "contract speed") by some small percent, the fly weights will be limited sufficiently outward to trip an overspeed switch and release a latch which allows a jaw to grip the governor rope and arrest its motion. The arrested governor rope causes actuators to pull safety rods on the elevator car causing the operation of safety brakes (sometimes called "safeties"), which are typically wedges that become jammed between a safety block and opposite sides of the elevator guide rail causing an increasing frictional force which abruptly stops the elevator.
A 1907 German patent, No. 198,255, suggested using electromagnets as an elevator safety brake, which would engage as a result of cable breakage, slackening of cable tension or exceeding determined speeds. Braking action is due both to mechanical friction and electromotive force generated in the car's guidance rail. A battery is used, and the operational capability of the system is tested with a switch each time that the elevator comes to rest. Similar eddy current braking systems have been devised for railroad trains, one example of which is shown in a pamphlet entitled "Eddy Current Brake WSB", published by Knorr-Bremse GMBH, 1975. The system described therein has electromagnets of alternating polar orientation dispersed above a length of track, on a carrier which hangs directly from the railway car truck. The magnets are kept suspended away from the rails by pneumatic cylinders except when emergency braking is desired; then, the air pressure is released so that the brake can drop down on the rail, thereby providing frictional braking action as a consequence of the electromagnetic attraction of the electromagnets to the rail, as well as magnetodynamic braking as a consequence of eddy currents induced by the alternating magnetic poles traversing the material of the track.
The safety codes which are imposed by a variety of governments frequently require that all safety devices must be fail-safe, that is, operative without electrical current and in the absence of electrical signals. This would render the use of electromagnets for an eddy current safety brake inadequate under such codes. Additionally, some codes require that safeties
(such as the upwardly-pulled wedges referred to hereinbefore) be bi-directional, thereby capable of arresting run away upward motion as well as runaway downward motion of an elevator car.